Field
Embodiments of the disclosure relate generally to the field of area control of jet engine nozzle exhaust and more particularly to embodiments for inducing flow separation in the divergent section of an exhaust nozzle to symmetrically alter the effective divergence angle of the nozzle walls to alter effective exit area.
Background
Exhaust nozzle exit area (A9) control for jet engines enhances engine and aircraft performance. With additional requirements for increased maneuverability and performance of modern jet aircraft as well as survivability requirements, fixed geometry nozzle systems which provide for exit area control including vectored thrust systems have become important in achieving overall performance goals. Exit area control allows tailoring of engine performance for thrust optimization. Mechanical systems often use deflecting surfaces to physically alter nozzle shape and area. Mechanical control of the throat area has been attempted before (see U.S. Pat. No. 2,846,843 to Clark et al, entitled “Variable area convergent-divergent exhaust nozzle and control therefor”) which does control the expansion ratio, but with a resulting change in the nozzle flow rate.
Fluidic systems have been employed but typically affect nozzle throat area or result in the generation of shocks in the divergent section which may be undesirable. Fluidic throat area control has been performed by as disclosed in U.S. Pat. No. 5,996,936 to Mueller entitled “Fluidic throat exhaust nozzle”, and suffers the same problem of nozzle flow rate variation with a change in expansion ratio.
It has been attempted to control A9 with layers of combustible material which burn off during flight to give variable A9. See U.S. patent application Ser. No. 09/942,238 to Hawkins and Murdock entitled “Combustible outgassing material lined altitude compensating rocket nozzle”. However, it is not always desirable to have combustion occurring on the walls of a nozzle. Nor do combustibles allow cyclic changes of area control during a flight mission as the combustibles can only be used once.
A combined system as disclosed in U.S. Pat. No. 3,010,280 to Hausmann et al entitled “Variable-expansion nozzle” employs blowing combustible mixtures into the divergent section to occupy flow area, thus reducing the overall nozzle exit area. Again, it is not always permissible to use combustibles near the walls of a nozzle due to material limitations.
Mechanical systems are heavy due to the requirements for large control surfaces and actuators. Large amounts of injected flow in fluidic systems are not preferable due to the performance impact on the engine to supply the large amounts of secondary flow for injection (flow that could otherwise be used to produce thrust).
It is therefore desirable to avoid the weight penalties of mechanical nozzle exit area adjustment systems by providing effective exit area control. It is also desirable to provide effective exit area control which does not impact the nozzle throat area, thus easily maintaining the engine mass flow. Additionally, it is desirable to provide effective exit area control which is simple to implement and minimizes thrust losses.